The present invention relates generally to semiconductor lasers. More particularly, the present invention relates to heterojunction active regions of semiconductor lasers.
Semiconductor lasers have become more important. One of the most important applications of semiconductor lasers is in communication systems where fiber optic communication media is employed. With growth in electronic communication, communication speed has become more important in order to increase data bandwidth in electronic communication systems. Improved semiconductor lasers can play a vital roll in increasing data bandwidth in communication systems using fiber optic communication media. A preferred component for optical interconnection of electronic components and systems via optical fibers is a semiconductor laser known as a vertical cavity surface emitting laser (VCSEL). Referring to FIG. 1A, a prior art vertical cavity surface emitting laser (VCSEL) 100 is illustrated. VCSEL 100 is cylindrical in shape and includes heterojunctions of semiconducting materials. When VCSEL 100 is lasing, a laser light is emitted from the top surface in a region defined by the optical confinement region 103. When lasing, the VCSEL 100 has a transverse mode field 108 and a longitudinal mode field 109. VCSEL 100 includes a first terminal 101 and a second terminal 102 coupled respectively to the top and bottom surfaces of the VCSEL to provide current and power. VCSEL 100 includes distributed Bragg reflector (DBR) layers 104A and 104B defining the optical confinement region 103. The optical confinement region 103 provides optical confinement such that the light can be reflected between the DBR layers 104A and 104B in a reinforcing manner to provide light amplification. The optical confinement region 103 is a cylindrical region having a diameter of D. VCSEL 100 includes heterojunction semiconductor layers 105. The region of the heterojunction semiconductor layers 105 which overlaps with the optical confinement region is referred to as the active region 106. The active region 106 provides current confinement so as to provide lasing when a threshold current is supplied to the VCSEL 100. Threshold current is the current level required for injecting enough carriers (electrons and holes) for lasing to occur. The frequency and wavelength of the light emitted from a VCSEL is a function of the structure and composition of materials of the active region 106. The typical structure of the heterojunction semiconductor layers 105 and active region 106 includes one or more pairs of quantum wells. Quantum wells are formed from joining particular semiconductor materials having a thickness of around one hundred Angstroms or less where the quantum confinement effects become important. Under quantum confinement effects, the effective bandgap of the quantum well material increases with decreasing thickness. Surrounding the one or more pairs of quantum wells are semiconductor materials that provide cladding and barriers. Quantum wells are key components in laser diodes in that they can strengthen electro-optical interactions by confining carriers to small regions.
To improve optical confinement, index guiding may be used. Index guiding uses layers of different compounds and structures to provide a real refractive index profile to guide the light. Alternatively a VCSEL may be gain guided. In gain guiding, the carriers induce a refractive index difference which is a function of the laser current level and output power.
VCSELs emit a coherent light beam of light from the surface of the wafer in which they are fabricated owing to the presence of the vertical optical cavity (the optical confinement region 103) formed by the two distributed Bragg reflectors (DBR) 104A and 104B on either side of a gain region (the active region 106). VCSELs are most often formed by epitaxial growth of the DBRs 104A and 104B and the active region 106 on a single crystal semiconductor wafer. In a VCSEL, the number of defects in its layers is minimized when they are grown on a substrate with the same lattice constant and crystal structure. To manufacture a VCSEL in this manner, there are a limited number of materials that can be incorporated into VCSEL devices that have the high performance properties required for lasing. The DBRs 104A and 104B in VCSEL 100 typically consists of alternating quarter wavelength thick layers of materials with different indices of refraction. The larger the difference in index of refraction, the fewer will be the number of layers required to achieve the necessary reflectivity. Gallium-Arsenide/Aluminum-Gallium-Arsenide (GaAs/AlGaAs) materials have to present been the most successful for use in VCSEL structures due to their wide differences in refractive indices. While GaAs/AlGaAs materials are the preferred materials for the DBRs 104A and 104B, they are not suitable for fabricating an active region 106 providing photon emission at longer wavelengths, such as the 1.3 xcexcm or 1.55 xcexcm wavelength standards that are well suited to optical fiber loss and dispersion minima. Heretofore, there have been no suitable materials or composition of materials in which to fabricate VCSELs at these longer wavelengths such as the 1.3 xcexcm or 1.55 xcexcm wavelength standards.
Prior art attempts to fabricate 1.3 xcexcm VCSELs on GaAs have focused on replacing the active region 106 with new heterojunction semiconductor layers 105 of material that emit photons at wavelengths of 1.3 xcexcm. These attempts include providing Gallium-Arsenide-Antimonide (GaAsSb) quantum wells, Gallium-Indium-Nitrogen-Arsenide (GaInNAs) quantum wells, or Indium-Arsenide (InAs) or Indium-Gallium-Arsenide (InGaAs) quantum dot active regions. In the case of the GaAsSb quantum wells, the thickness of the quantum well layer and its Antimony (Sb) composition need to be maintained below the elastic strain limit to avoid defect formation. This constraint also limits the achievable photon emission wavelength to less than the desired 1.3 xcexcm. GaInNAs quantum wells can in principle be grown to emit at 1.3 xcexcm with arbitrary thicknesses on GaAs substrates, if, the Indium (In) and Nitrogen (N) compositions can be controlled to match the lattice constant of GaAs. However, the limitation in Nitrogen (N) composition has limited the photon emission wavelength to less than the desired 1.3 xcexcm. Regarding InAs or InGaAs quantum dot active regions, current manufacturing is unable to achieve high enough density of dots in order to sustain laser operation in a VCSEL.
Increased strain in the materials of the heterojunction 105 and active region 106 of semiconductor lasers has been used to reduce threshold currents and optical losses. Strain is generally produced by changing the concentration ratio of atoms forming the materials of the heterojunction 105. There are two types of strain in the materials, tensile strain and compressive strain. Cladding layers of the quantum well are usually lattice matched to the substrate with respect to strain while the barrier layers may have the opposite strain of the quantum well layer. Strained layer quantum well structures may be employed to try to achieve longer wavelength emission of photons. However, with the increased strain in the quantum well, there is a growth in lattice mismatched active regions which presents limits to the achievable wavelength ranges. For example, consider InGaAs or GaAsSb quantum wells used alone as the active region, which are limited to a laser emission wavelength between 1.1 and 1.25 xcexcm respectively. As the In or Sb composition is increased the layer thickness must be decreased in order to remain within the elastic strain limit. Unfortunately, the quantization energy in the quantum well increases approximately quadratically as the well thickness decreases and thus the emission wavelength saturates. Recently the possibility of using strain compensation and graded quantum wells to extend the emission wavelength of InGaAs quantum wells has been explored. However, this approach only enabled the extension of the emission wavelength to 1.2 xcexcm. Further increases in the photon emission wavelength using this approach appear unfeasible.
It is desirable to overcome the limitations in the prior art and provide new heterojunction semiconductor layers of material that can provide an active region which can lase at a wavelength of 1.3 xcexcm or more.
Briefly, the present invention includes a method, apparatus and system as described in the claims. A W quantum well structure is provided by the present invention to enhance the active region of semiconductor lasers in order to provide long wavelength emission of photons. The W quantum well structure is expanded to an alternate embodiments of Type II multiple quantum wells. The present invention provides an active region, incorporating the W quantum well structure, for semiconductor lasers with energy band lineups in heterostructures to achieve emission at wavelengths of 1.3 xcexcm or greater. Application of the active region to a basic laser diode device, a VCSEL, and exemplary materials are disclosed which can be used to achieve lasing at wavelengths of 1.3 xcexcm and greater. The active region is comprised of one or more sets of triad layers of GaAs1xe2x88x92xNx/GaAs1xe2x88x92ySby/GaAs1xe2x88x92xNx to provide the W quantum well structure. The energy band of these materials provides a staggered band alignment which causes electrons and holes to be confined in adjacent layers to one another. Because the wavefunctions associated with these materials tunnel into the adjacent layers, optical emission at a longer wavelength is achievable than otherwise available from the energy gaps of the constituent materials alone.